The present invention relates to use of a bimodal particle size distribution of scouring particles in a process for cooling a hot gaseous suspension of pigmentary metal oxides in an externally cooled conduit.
In producing pigmentary TiO.sub.2 by oxidizing TiCl.sub.4 in the vapor phase at from about 900.degree. to 1600.degree. C. with an oxygen-containing gas, a hot gaseous suspension of TiO.sub.2 solids and free chlorine must be quickly cooled below 600.degree. C. within about 1-60 seconds following discharge of the suspension from a reactor. This cooling is accomplished in a conduit, i.e., a flue so that undesired TiO.sub.2 particle size growth is prevented and particle agglomeration is minimized. Pigment product properties such as carbon black undertone (CBU) and gloss are a function of primary particle size and particle agglomeration, respectively.
The conduit is generally cooled externally with flowing water. The fine TiO.sub.2, however, has a strong tendency to deposit in adherent layers on the inner walls of the conduits. Good heat transfer can be maintained by minimizing the adherent layers by adding scouring particles, i.e., scrubs. For example, U.S. Pat. No. 2,721,626 solves this surface coating problem by introducing relatively dense, hard abrasive particles into the hot suspension. The specific scrubs mentioned are sand, sintered particles of titanium dioxide, mullite or refractory alumina particles. The particle size range disclosed is from 100 mesh to 1/4 inch. No mention is made of particle size distribution of the added scrubs. U.S. Pat. No. 3,511,308 provides a further improvement by adding a particulate, anhydrous, water-soluble salt having a melting point above about 700.degree. C. and a hardness on the Mohr scale not greater than about 5. The particle size range disclosed is from 100 mesh to 4 mesh. Again, no mention is made of particle size distribution of the added scrubs.
This invention is yet another advance in a process for cooling a hot suspension of fine solids in a gas wherein the suspension is passed through an externally cooled conduit. There is a need to further improve the pigment properties such that having an average primary particle size from about 0.05 to about 0.5 microns produces a desirable CBU and minimizing agglomeration results in increased gloss. There is a concomitant need to maintain higher production rates without quality loss. The present invention meets these needs.